Conventionally, navigation devices which calculate a traveling distance of a vehicle by using vehicle speed pulses are disclosed (e.g., see JP2647342B). The navigation device of JP2647342B acquires external signals from a wheel speed sensor, a gyroscope and a GPS receiver. These external signals are transmitted to a position detecting module of the navigation device. The position detecting module calculates a traveling distance of a vehicle as follows. The position detecting module applies an azimuth detected by the gyroscope to a vehicle speed pulse count detected by the wheel speed sensor to calculate a quadrature component of the vehicle speed pulse count. Based on a position of the vehicle detected by the GPS receiver, the position detecting module calculates a linear distance by which the vehicle has traveled. The position detecting module then compares the quadrature component with the linear distance to calculate a pulse distance coefficient (scale factor). Finally, the position detecting module calculates the traveling distance of the vehicle by using a product of the pulse distance coefficient and the vehicle speed pulse count.
Moreover, azimuth calculating devices which calculate an azimuth of a vehicle are conventionally disclosed (e.g., see JP4734329B). The azimuth calculating device of JP4734329B calculates an azimuth of a vehicle as follows. The azimuth calculating device multiplies an output value of an angular velocity sensor by a gain value to calculate an angular velocity before correction. The azimuth calculating device then multiplies the angular velocity before correction by a correction coefficient to calculate a corrected angular velocity of the vehicle. Finally, the azimuth calculating device calculates the azimuth of the vehicle based on the angular velocity of the vehicle. The correction coefficient is updated by comparing the calculated azimuth of the vehicle with an azimuth obtained by using a GPS.
The navigation device of JP2647342B calculates the scale factor generally by dividing the traveling distance of the vehicle by the vehicle speed pulse count. However, when the vehicle accelerates or decelerates, since a drive shaft of the vehicle is distorted and/or the diameters of wheels are varied due to load fluctuation on the wheels, the relationship between the traveling distance of the vehicle and the vehicle speed pulse count changes. Therefore, in the navigation device of JP2647342B, when the vehicle accelerates or decelerates, an error may be caused in the scale factor and, further, an error may be caused in the traveling distance and the speed of the vehicle which are calculated based on the vehicle speed pulse count.